1. Field of the Invention
The present invention relates to a thin film transistor including low resistance conductive thin films and a manufacturing method thereof.
2. Description of the Background Art
It has been known for many years that oxides such as zinc oxide or magnesium zinc oxide have excellent characteristics as a semiconductor (an active layer). In recent years, active research and development of a semiconductor thin film layer using these compounds have been made in order to apply such a semiconductor thin film layer to electronic devices such as a thin film transistor (hereinafter abbreviated as TFT), a light emitting device, and a transparent conductive film.
An oxide TFT including a semiconductor thin film layer made of zinc oxide or magnesium zinc oxide has greater electron mobility and better TFT characteristics than an amorphous silicon TFT including a semiconductor thin film layer of amorphous silicon (a-Si:H), which has been mainly used for liquid crystal displays. Another advantage of the oxide TFTs is that high electron mobility can be expected because a crystalline thin film is formed even at a temperature as low as a room temperature. These advantages have been encouraging the development of the oxide TFTs.
TFTs using an oxide semiconductor thin film layer, such as a bottom gate type TFT (see, for example, Japanese Patent Publications No. 2005-033172 and No. 2004-349583) and a top gate type TFT, have been reported.
The bottom gate type TFTs include, for example, a lamination of a gate electrode disposed over a substrate, a gate insulating film, source/drain electrodes, and an oxide semiconductor thin film layer, which are laminated in this order.
On the other hand, the top gate type TFTs, for example, include a lamination of source/drain electrodes disposed over a substrate, an oxide semiconductor thin film layer, a gate insulating film, and a gate electrode, which are laminated in this order.
In both of the bottom and top gate type TFTs, sufficient contact is required between each of the source/drain electrodes and the oxide semiconductor thin film layer (primarily comprising zinc oxide and the like) in order to ensure high current drive power.
In a conventional method, a source/drain region having lower resistance than the oxide semiconductor thin film layer is provided to improve the contact property between the source/drain electrodes and the oxide semiconductor thin film layer.
FIG. 9A shows a TFT 500 as one example of the TFTs provided according to the conventional method. The TFT 500 has a pair of low resistance conductive thin films 110 sandwiched between the oxide semiconductor thin film layer 103 and a pair of source/drain electrodes 102 placed on a substrate 101. Since the low resistance conductive thin films 110 have a lower resistance than the oxide semiconductor thin film layer 103, they improve the contact between each of the source/drain electrodes 102 and the oxide semiconductor thin film layer 103. The oxide semiconductor thin film layer 103 is disposed on the low resistance conductive thin films 110 and on an area of the substrate 101 exposed between the pair of low resistance conductive thin films 110, while the outer periphery 110a (See FIG. 9B described below for a plan view) of the low resistance conductive thin films 110 remains uncovered. All the exposed surfaces of the oxide semiconductor thin film layer 103 are covered with a gate insulating film 104. A gate electrode 106 is disposed over the gate insulating film 104. FIG. 9B is a plan view of an array of the TFTs 500 shown in FIG. 9A. In FIG. 9B, two of the TFTs 500 are aligned in parallel. FIG. 9A is a cross sectional view along line IXA-IXA of FIG. 9B. For clarity, FIG. 9B omits gate insulating film 104 shown in FIG. 9A
In manufacturing the TFT 500, first a pair of source/drain electrodes 102 is patterned and then the low resistance conductive thin film 110 is formed. The low resistance conductive thin film 110 is separated into a plurality of low resistance conductive thin films 110 that are spaced apart from each other, using a photo-lithography technique. Accordingly, an outer periphery 110a (cross-hatched in FIG. 9B) of the low resistance conductive thin films 110 protrudes from the outer profile of the oxide semiconductor thin film layer 103. As shown in FIG. 9B, at least a distance D (distance D=width A+gap B+width A) is needed between the oxide semiconductor thin film layers of the TFTs. A narrower distance D is preferable in order to achieve higher integration of TFTs. The width A is defined by the mask-alignment accuracy of an aligner, in other words, by the alignment accuracy in the photo-lithography of the low resistance conductive thin film 110 and the oxide semiconductor thin film layer 103. The higher the alignment accuracy is, the smaller the width A becomes. On the other hand, the gap B is defined by the minimum resolution during the patterning of the low resistance conductive thin film 110. The higher the minimum resolution is, the smaller the gap B becomes. When a conventional aligner for an LCD is used, the width A determined by the alignment accuracy, is about 1.5 μm, and the gap B determined by the minimum resolution is about 4.0 μm. Therefore, in the conventional TFT 500, the distance D between the oxide semiconductor thin film layers 103 is approximately 7.0 μm (1.5 μm+4.0 μm+1.5 μm) (see FIG. 9B).
On the other hand, in manufacturing a TFT that includes no low resistance conductive thin film, an oxide semiconductor thin film layer is laid over the source/drain electrodes of a plurality of TFTs, and then the oxide semiconductor thin film layer is patterned. Therefore, the width A required in TFT 500 according to the mask-alignment accuracy is not necessary. Thus the width A is eliminated from the distance D so that the distance D includes only the gap B.
As mentioned above, for TFTs including no low resistance conductive thin film, the minimum distance D between the adjacent oxide semiconductor thin film layers is equal to the gap B, whereas, for TFTs (e.g. TFT 500) including the low resistance conductive thin film, the minimum distance D between the oxide semiconductor thin film layers is equal to the sum of width A, gap B, and width A (width A+gap B+width A). In other words, in the TFTs (e.g. TFT 500) including the low resistance conductive thin film for improving the contact properties, the low resistance conductive thin film 110 forces the gap between the oxide semiconductor thin film layers to be wider, which results in difficulty in achieving a high integration of the TFTs.